1. Field of the Invention
The present invention relates to artificial lift for hydrocarbon wells. More particularly, the invention relates to gas operated pumps for use in a wellbore. More particularly still, the invention relates to a method and an apparatus for improving production from a wellbore.
2. Background of the Related Art
Throughout the world there are major deposits of heavy oils which, until recently, have been substantially ignored as sources of petroleum since the oils contained therein were not recoverable using ordinary production techniques.
These deposits are often referred to as “tar sand” or “heavy oil” deposits due to the high viscosity of the hydrocarbons which they contain. These tar sands may extend for many miles and occur in varying thicknesses of up to more than 300 feet. The tar sands contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount, which ranges from about 5 to about 20 percent by weight of hydrocarbons. Bitumen is usually immobile at typical reservoir temperatures. Although tar sand deposits may lie at or near the earth's surface, generally they are located under a substantial overburden or a rock base which may be as great as several thousand feet thick. In Canada and California, vast deposits of heavy oil are found in the various reservoirs. The oil deposits are essentially immobile, therefore unable to flow under normal natural drive or primary recovery mechanisms. Furthermore, oil saturations in these formations are typically large which limits the injectivity of a fluid (heated or cold) into the formation.
Several in situ methods of recovering viscous oil and bitumen have been the developed over the years. One such method is called Steam Assisted Gravity Drainage (SAGD) as disclosed in U.S. Pat. No. 4,344,485, which is herein incorporated by reference in its entirety. The SAGD operation requires placing a pair of coextensive horizontal wells spaced one above the other at a distance of typically 5-8 meters. The pair of wells is located close to the base of the viscous oil and bitumen. Thereafter, the span of formation between the wells is heated to mobilize the oil contained within that span by circulating steam through each well at the same time. In this manner, the span of formation is slowly heated by thermal conductance.
After the oil in the span of the formation is sufficiently heated, the oil may be displaced or driven from one well to the other establishing fluid communication between the wells. At this point, the steam circulation through the wells is terminated and steam injection at less than formation fracture pressure is initiated through the upper well while the lower well is opened to produce draining liquid. As the steam is injected, a steam chamber is formed as the steam rises and contacts cold oil immediately above the upper injection well. The steam gives up heat and condenses; the oil absorbs heat and becomes mobile as its viscosity is reduced allowing the heated oil to drain downwardly under the influence of gravity toward the lower well.
The steam chamber continues to expand upwardly and laterally until it contacts an overlying impermeable overburden. The steam chamber has an essentially triangular cross-section as shown in FIG. 2A. If two laterally spaced pairs of wells undergoing SAGD are provided, their steam chambers grow laterally until they make contact high in the reservoir. At this stage, further steam injection may be terminated and production declines until the wells are abandoned.
Although the SAGD operation has been effective in recovering a large portion of “tar sand” or “heavy oil” deposits, the success of complete recovery of the deposits is often hampered by the inability to effectively move the viscous deposits up the production tubing. High temperature, low suction pressure, high volume with a mixture of sand are all characteristics of a SAGD operation.
Various artificial lift methods, such as pumps, have been employed in transporting hydrocarbons up the production tubing. One type of pump is the electric submersible pump (ESP), which is effective in transporting fluids through the production tubing. However, the ESP tends to gas lock in high temperature conditions. Another type of pump used downhole is called a rod pump. The rod pump can operate in high temperatures but cannot handle the large volume of oil. Another type of pump is a chamber lift pump, commonly referred to as a gas-operated pump. The gas-operated pump is effective in low pressure and low temperature but has low volume capacity. An example of a gas-operated pump is disclosed in U.S. Pat. No. 5,806,598, which is incorporated herein by reference in its entirety. The '598 patent discloses a method and apparatus for pumping fluids from a producing hydrocarbon formation utilizing a gas-operated pump having a valve actuated by a hydraulically operated mechanism. In one embodiment, a valve assembly is disposed at an end of coiled tubing and may be removed from the pump for replacement. Generally, if a SAGD well is not operated efficiently by having an effective pumping system, liquid oil will build in the steam chamber encompassing both the lower and the upper wellbores. If the oil liquid level rises above the upper wellbore and remains at that level, a large amount of oil deposit remains untouched in the reservoir. Due to this problem many wells using the SAGD operation are not recovering the maximum amount of deposits available in the reservoir.
Several other recovery methods have problems similar to a SAGD operation due to an inadequate pumping device. For example, cyclic steam drive is an application of steam flooding. The first step in this method involves injecting steam into a vertical well and then shutting in the well to “soak,” wherein the heat contained in the steam raises the temperature and lowers the viscosity of the oil. During the first step, a workover or partial workover is required to pull the pump out past the packer in order to inject the steam into the well. After the steam is injected, the pump must than be re-inserted in the wellbore. Thereafter, the second step of the production period begins wherein mobilized oil is produced from the well by pumping the viscous oil out of the well. This process is repeated over and over again until the production level is reduced. The process of removing and re-inserting the pump after the first step is very costly due to the expense of a workover. In another example, continuous steam drive wells operate by continuously injecting steam downhole in essentially vertical wells to reduce the viscosity of the oil. The viscous oil is urged out of a nearby essentially vertical well by a pumping device. High temperature, low suction pressure, and high pumping volume are characteristics of a continuous steam drive operation. In these conditions, the ESP pump cannot operate reliably due to the high temperature. The rod pump can operate in high temperature but has a limited capacity to move a high volume of oil. In yet another example, methane is produced from a well drilled in a coal seam. The recovery operation to remove water containing dissolved methane is often hampered by the inability of the pumping device to handle the low pressure and the abrasive material which are characteristic of a gas well in a coal bed methane application.
There is a need, therefore, for an improved gas operated pump that can effectively transport fluids from the horizontal portion of a SAGD well to the top of the wellbore. There is a further need for a pump that can operate in low pressure and high temperature conditions with large volume capacity. There is yet another need for a pump that can remain downhole during a cyclic steam drive operation. Furthermore, there is a need for a pump that can operate in low pressure conditions and handle abrasive materials. There is also a final need for a pump to operate in a wellbore where there is no longer sufficient reservoir pressure to utilize gas lift in order to transport the fluid to the surface.